Time-lag fuses



F. J. KOZACKA TIME-LAG FUSES June 13, 1961 2 Sheets-Sheet l Filed Sept.30, 1958 almma ma;

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June 13, 1961 F, J, KOZACKA 2,988,620 A TIMELAG FUSE-S Filed sept. 5o,1958 2 sheets-sheet 2 SECS. m O LLI (D ln Q .Q5. Y

nted States Patent C) 2,988,620 TIME-LAG FUSES Frederick J. Kozacka,South Hampton, N.H., assignor to The Chase-Shawmut Company, Newburyport,Mass. Filed Sept. 30, 1958, Ser. No. 764,293 7 Claims. (Cl. 200-1131)This invention relates to electric fuses for the protection of electriccircuits against excessive current llow, and more particularly toelectric fuses required to have a substantial time-lag in theovercurrent range and required to blow very rapidly in the short-circuitcurrent range.

It is one object of this invention to provide electric fuses which havea substantial time-lag inthe overcurrent range without resorting torelatively large'heat-absorbing metal masses attached to the fusibleelement, or fuse link, frequently designated as lag-blocks.

Another object of the invention is to provide electric fuses comprisinga ribbon type fuse link which has a point of reduced cross-section tocause formation of a break at the occurrence of excess currents, whichpoint of reduced cross-section is adapted to form a break by metalcorrosion relatively slowly if the excess current is relatively smalland to form a break by metal corrosion relatively rapidly if theexcesscurrent is` relatively large.

Another object of the invention is to provide electric fuses of thetime-lag type wherein the fusible element is made of a high conductivityhigh fusing point metal and is provided with an overlay of a low fusingpoint metal which destroys the base metal of which the fusible elementis made by metal corrosion when the overlay metal reaches the fusingpoint thereof, in which fuses fusion of the overlay metal is delayed .onoccurrence of relatively small overcurrents by the presence of arelatively large area of heat .exchange between the base metal and theoverlay metal.

A further object of vthe invention is to provide a mechanical connectionfor connecting the fusible element or fuse link into an electric circuitwhich connection has a minimum of ohmic resistance and therefore tendstoward achieving of a cool running fuse.

Still another object of the invention is to provide novel and improvedmethods of manufacturing so-called dual element fuses.

Other objects and advantages of the invention will become apparent asthis specification proceeds, and the features of novelty whichcharacterize the invention will be pointed out with'particularity in theappended claims forming part of this specification.

For a better understanding of the invention reference may be had to theaccompanying drawings in which:

. FIG. l is a longitudinal section of a fuse embodying the inventiontaken along 1-1 of FIG. 2;

FIG. 2 isV a longitudinal section of the fuse shown in FIG. ltaken'along 2-2 of FIG. l;

FIG. 3 shows a detail of FIG. 2 on a considerably larger scale; Y

FIG. 4 shows time-temperature characteristics illustrating the theoryunderlying the present invention;

FIG. 5 is a section taken along 5--5 of FIG. 2;

FIG. 6 is a cross-section of a modied link structure capable ofachieving even longer rtime-lags than the structure of FIGS. 1,2 and 3;

FIG. 7 is a top plan view of the structure shown in l FIG. 6; and

blade contacts 3 are conductively interconnected by fusi' ble elements 4in the form of a pair of relatively wide copper ribbons. The width w ofribbons 4 is almost equal to the width W of blade contacts 3. Pins 5project transversely through caps 2 and casing 1 into bores which areprovided in knife blade contacts 3 and extend parallel to the generalplane of contacts 3. The axially outer ends of ribbons 4 are clampedagainst blade contacts 3 by pressure plates 6 and clamping rivets 7. Ascan best be seen in FIG. 5 blade contacts 3 are sandwiched betweenribbons 4 and each ribbon 4 is sandwiched between a blade contact 3 andone of pressure plate 6. 'Ihree rows 8 of circular perforations 9project transversely across ribbon 4. Pairs of contiguous perforationsform therebetween necks 10, i.e. points of greatly reduced crosssectional area where the current density is very high when the ribbon 4carries electric currents. Reference numeral 11 has been applied toindicate an overlay on ribbon 4 made of a relatively low fusing pointmetal capable, upon fusion thereof, of destroying the base metal ofribbon 4 by corrosion. The base metal 4 is a high conductivity highfusing point metal such as copper or silver, and the overlay metal maybe tin, an alloy of tin, indium, an alloy of indium, etc. In the case ofa silver ribbon 4 an overlay of indium, or an alloy of indium, may beused. Where the base metal is copper the overlay is preferably of tin,or an appropriate alloy of tin. In the embodiment of the invention shownin FIGS. l and 2 only the axially inner row 8 of perforations 9 isassociated with an overlay 11 of a low fusing point metal, and theaxially outer rows 8 of perforations are not associated with such anoverlay.

Referring now more particularly to FIG. 3 which shows the row ofperforations 8 associated with an overlay v11 of a low fusing pointmetal, it will be apparent that the overlay extends from the right sideof row 8 exactly to the middle of necks 10 but not beyond necks 10. Thisfact is of considerable importance because the intended mode ofoperation of the fuse depends upon it as will become more apparent asthis specilication proceeds. In order to obtain the aforementionedprecise distribution or geometry of the overlay 11 the portion of ribbon4 to the right of the middle row `8 of perforations 9 is heated. As aresult the temperature to the right of middle row 8 is high but thetemperature to the left of middle row 8 remains relatively low sincenecks 10 operate as heat dams. In other words, to the left of necks 10heat dissipation is so intense as to result in a very steep temperaturegradient at the necks 10. To the right of necks 10 the temperature is sohigh that tin or another low fusing point metal used instead of tin toform the overlay 11 is in uid state. Slightly to the left of necks 10the temperature of necks 10 is below the fusing point of tin, or itsequivalent base metal corroding agent, and therefore overlay-formationstops abruptly at the points of steep temperature gradient which areidentical with the points 10 of minimum crosssection of link 4. Whenlink 4 is completed the entire link is submersed in a pulverulentarc-quenching filler, e.g. gypsum powder, to quench the arcs formed atthe points of break 10 when the fuse blows.

At relatively low but protracted overloads initial link destructionoccurs only at the middle row 8 of perforations 9, and arc extinction iseffected by back-burning from that row of perforations. Link destructionunder the action of relatively small protracted overloads involvessubstantial timerlags. At relatively high excess currents linkdestruction is initiated at the two axially outer rows 8 of perforations9 and is rapidly followed by link destruction at the middle row 8 ofperforations 9, thus forming a third series break. It is apparent fromthe foregoing that the middle row 8 of perforations 9 forms circuitinterrupting breaks both at the occurrence of relatively smalrlprotracted overloads and at the occurrence of relatively high excesscurrents in the nature of short-circuit currents. There is a shift ofthe point of initial link destruction when the hottest point of overlay11 reaches the fusing point of the low fusing point metal at the sametime as the point or points of narrowest cross-section o-f the linkwhich are not associated with a corrosive overlay reach the fusing pointof the base metal. In other words, there is a critical excess currentwhich, when exceeded, results in shifting of the point of initial linkdestruction from the hottest point of link 4 associated with overlay 11to the point or points of narrowest cross-section of link 4 notassociated with a corrosive overlay.

The longer it takes necks which are associated with overlay 1l to reachthe fusing temperature of the low fusing point metal of which overlay 11consists, the greater the time-delay of the particular fuse. The rate ofincrease of temperature at the necks 10 is greatly reduced by the largearea of heat exchange between the overlay metal and the base met-al. Inother words, since the overlay metal starts at necks 10 and is spreadacross the entire width of the fuse link, and since the fuse link isapproximately as wide as the blade contacts 3, the abstraction of heatfrom the necks or points of reduced cross-section 10 is highlyeffective, and thus delays the time required by a given overload currentto heat the overlay metal to its fusing point. If the overlay metal weremore concentrated, or lumped, the surface of heat exchange with the basemetal of link 4 would be relatively smaller, and the heat ow away fromnecks 10 relatively less. Hence the times required for necks 10 to reachthe fusing point of the overlay met-al would be relatively longer.

It is thus apparent that the time-delays which can be achieved with astructure of the above character are due predominantly to the presenceof a large area of heatexchange between the fuse link proper and the lowfusing `point overlay thereon rather than to the presence of arelatively large heat absorbing mass of metal on the fuse link.

In FIG. 4 the `temperature at a point of a fuse link covered by acorrosive overlay of a low fusing point metal has been plotted againsttime, assuming the link to be heated by a relatively small but excessiveoverload current. FT is the fusing temperature of 'the low fusing pointmetal at which corrosion of the base metal becomes significant. If theheat abstraction from a neck which is covered by the overlay isrelatively small, the ternperature at the neck rises relatively rapidly,as indicated by the exponential curve E, and fusion of the overlay metaltakes place after a relatively short period of time t.

yOn the other hand, if heat abstraction from a neck covered by theoverlay is relatively large, the temperature at the neck risesrelatively slowly, Aas indicated by the exponential curve e, and fusionof the overlay metal takes place after a relatively long period of timeT.

The geometry of the clamping means 6, 7 for conductively securing links4 to blade contacts 3 is an outgrowth of the nature of the fusibleexcess current protective means 4, 9, 11. The fusible excess currentprotective means call for a relatively large `width W of link ribbons 4substantially equal to that of blade contacts 3. This, in turn, resultsin a relatively small current-density at the area of contact betweenlink ribbons 4 and blade contacts 3. The smallness of that currentdensity makes it possible to dispense with solder joints and to resortto the simple clamp-type connections described and illustrated.

Where it is desired to achieve delay times in the overload range longerthan those that can be achieved with the overlay 11 shown in FIGS. l to3, inclusive, the link 4 may be covered on both sides with a destructivelow fusing point overlay 11. This has been shown in FIGS. 6 and 7. Theoverlay 11 shown in these two figures is identical to the overlay 11shown in FIGS. l, 2 and 3 and described in connection therewith, exceptthat 4 the overlay 11 shown in FIGS. 6 and 7 covers both sides ratherthan but one side of perforated copper ribbon 4.

In FIG. 8 reference letter A has been applied to indicate thetime-current-curve of a conventional one time fuse having a zinc link inlribbon form and reference letter B has been applied to indicate thetime-currentcurve of a fuse embodying the invention ysubstantiallyidentical with the struc-ture shown in FIGS. l, 2, 3 and 5. Both fuseshave the same voltage rating and current rating. It is apparent that thedelay times in the small overload range up to and beyond 5 times currentrating are considerably longer than the delay times of the prio-r artfuse though the fusible element of the prior art `fuse involves a muchlarger mass than the structure of FIGS. l, 2, 3 and 5. I-t was notpossible heretofore to achieve delay times of `the order represented bycurve B of FIG. 8 without adding relatively large masses to ribbon inselinks, known as lag-blocks, which are being dispensed with in fusesembodying the present invention.

The basic part of which fuses embodying this invention are made is arelatively wide copper riblbon 4, or a ribbon of another metal combiningthe properties of high fusing point and high conductivity, andrelatively low specic fusing energy, e.g. silver. At least onetransverse line 8 of perforations 9 forming therebetween a line ofpoints of reduced cross-section or necks 10 is being punched into ribbon4. Then ribbon 4 is being heated across the entire width thereof butonly to one side of the line of perforations 9. Heating is effected toand above the fusing temperature of the contemplated low fusing pointoverlay metal. While ribbon 4 is being heated at one side ofperforations 9, the temperature at the other side of perforations 9 isbeing maintained at a relatively low level, eig. at ambient temperature.Thereafter a pool of low fusing point metal is formed on ribbon 4 at theside thereof -Where heated, and coextensive with' the entire widththereof. The next step consists in causing the constituent metal of theaforementioned pool to flow up to but not beyond the line of points ofreduced crosssection 10. This may be achieved by tilting ribbon 4 in anappropriate -way to induce the melted metal thereon to flow under theaction of gravity in a direction longitudinally thereof. Finally ribbon4 is permitted to cool down to ambient temperature, resulting in thestructure illustrated in FIGS. l, 2 and 3.

While in the preferred embodiment of the invention illustrated in FIGS.l and 2 the fuse link 4 is relatively y wide and provided with aplurality of points of reduced cross-section, the fuse link 4 might berelatively narrow, as suggested by FIGS. 6 and 7, or even narrower thanshown in these two figures. Narrowing down the width of the link ispermissible where the fuse is intended to have a relatively smallcurrent-carrying capacity. In that particular instance the provision ofbut one single point of reduced cross-section or neck associated with anoverlay of a corrosive metal having a lower fusing point than the basemetal may suflice.

FIGS. 1 and 3 show the preferred geometry of my ribbon type fuse linkhaving a transverse line 8 of circular perforations 9. As a result thecross-section of link 4 decreases progressively in a directionlongitudinally thereof to a point of minimum cross-section, andincreases -with increasing distance from the aforementioned point ward aminimum and away from a minimum is, however, not a necessary requirementfor putting my invention into effect.

It will be understood that vthe axially outer points of aussage reducedcross-section of link 4 fuse faster at the occurrence of relativelylarge excess currents,Y such asshortcircut currents, than the axiallyinner points'of reduced cross-section cooled by overlay 11. lOn majorfaults involving a rapid rise of current the sequence of fusion of theaxially outer and axially inner points of reduced crosssection is veryrapid, and the heat exchange between, the axially inner points ofreduced cross-section andthe overlay 11 of relatively littlesignificance. 'Under-"such circumstances the axially inner pointsofjjreduced cross-section form a break or breaks while there is stillarcing at the axially outer points of reduced cross-section. In otherwords, the axially inner break cornes to the `help of the axially outerbreaks in generating arc voltage required for interruption of thefaulted circuit. Referring more particularly to FIG. 3, it is apparentthat each neck has a portion which is covered by overlay 11. To be morespecific, the portion left to the point of minimum cross-section is notcovered by overlay 11 and the portion to the right of the point ofminimum cross-section is covered by overlay 11. The portions of thenecks not covered and cooled by overlay 11 exhibit a sufliciently rapidrise in temperature when ribbon 4 is carrying shortcircuit currents toform a break in series with the breaks then formed by the axially outerlines or rows 8 of perforations 9. It will be apparent that the portionof ribbon 4 immediately to the left from center where there is nooverlay will exhibit a rise of temperature at the occurrence of currentsin the nature of short-circuit currents which is almost as fast as therise in temperature at the points of reduced cross-section formed by theaxially outer lines or rows 8 of perforations 9. There is, of course,some cooling from the overlay 11 to the left of center, or the medianplane of the fuse. Corrosion becomes significant in the median plane ofthe fuse or, in other words, at the left boundary line of overlay 11,when the fusing point of overlay 11 is reached by the portion of thelink 4 situated to the left of the median plane, or to the left of theleft boundary line of overlay 11. This explains why three breaks areformed in series at the occurrence of short-circuit currents, and whythe structure of FIG. 3 can operate either extremely slowly or extremelyfast, depending upon the intensity of the fault current involved. Whileit is generally desirable to provide points of reduced cross-section notassociated with a low fusing point metal adapted to form breaks veryrapidly on major faults in addition to the point or points of reducedcross-section associated with a low fusing point metal which formsbreaks either slowly or rapidly, depending upon the intensity of theexcess current, the first mentioned points of reduced cross-section maybe omitted if the circuit voltage of the circuit to be protected isrelatively low, say in the order of 250 volts, or less than 600 volts.

Since certain further changes can be made in the foregoing structuresand other embodiments can be made without departing from the spirit andscope of the invention, it is intended that all matter shown in theaccompanying drawings and described hereinbefore shall be interpreted asillustrative and not in a limiting sense.

I claim as my invention:

l. A fusible protective device comprising a ribbon of copper having apredetermined width, a plurality of circular perforations extendingtransversely across said ribbon forming therebetween a plurality oftransversely arranged points of minimal cross-section, said ribbon beingprovided with an overlay of a metal having a fusing point lower than thefusing point of copper and being substantially equal in width to saidpredetermined width of said ribbon, said overlay having a plurality ofprojections jutting into the gaps formed between said plurality ofperforations to said plurality of points of minimal crosssection but notsubstantially beynd said plurality of points of minimal cross-section.

2. A fusible protective device comprising a fuse link in ribbon form ofa relatively high conductivity high fusing point metal, a plurality oflines of perforations extending transversely across said link eachforming one transverse line of points of reduced cross-section, one ofsaid plurality of lines of perforations being associated with an overlayof a link-destroying relatively low fusing point metal coextensive withthe width of said link and projecting substantially to but notsubstantially beyond said line of points of reduced cross-section formedby said one of said plurality of lines of perforations.

3. A fusible protective device comprising a pair of fuse links in ribbonform made of a relatively high conductivity high fusing point metal, aline of perforations extending transversely across each of said pair oflinks forming a transverse line of points of reduced crosssection oneach of said pair of links, said line of perforations on each of saidpair of links being associated with an overlay of a link-destroyinglow-fusing-point metal coextensive with the width of each of said pairof links and projecting substantially to but not significantly beyondsaid line of points of reduced cross-section on each of said pair oflinks, a pair of knife blade contacts arranged at the axially outer endsof said pair of links having substantially the same width as said pairof links each sandwiched between said pair of links, and a pair ofpressure plates on each end of said pair of links coextensive with thewidth of each of said pair of links clamping said pair of links againstsaid pair of blade contacts.

4. A fusible protective device comprising a tubular casing of insulatingmaterial; a pair of terminal elements closing the ends of said casing; apair of blade contacts extending in a direction longitudinally of saidcasing each projecting through one of said pair of terminal elementsfrom the outside of said casing into the inside thereof; a ribbon of ametal having a relatively high conductivity and a relatively high fusingpoint, said ribbon having a width being approximately equal to the widthof said pair of blade contacts and said ribbon conductivelyinterconnecting the axially inner ends of said pair of blade contacts; aplurality of identical spaced parallel lines of perforations eachextending transversely across said ribbon and each defining a transverseline of necks of reduced cross-section forming parallel current paths;and an overlay of a metal on said ribbon having a relatively low fusingpoint and extending from an area of said ribbon adjacent one of saidtransverse lines of necks to said line of necks but not significantlybeyond said line of necks,

5. A fusible protective device as specified in claim 4 comprising a pairof ribbons arranged on opposite sides of said pair of blade contacts.

6. A fusible protective device comprising a tubular casing of insulatingmaterial; a pair of terminal elements closing the ends of said casing; apair of 4blade contacts extending in a direction longitudinally of saidcasing each projecting through one of said pair of terminal elementsfrom the outside of said casing into the inside thereof; a ribbon of ametal having a relatively high conductivity and a relatively high fusingpoint, said ribbon having a width approximately equal to the width ofsaid pair of blade contacts and said ribbon conductively interconnectingthe axially inner ends of said pair of blade contacts; said ribbonhaving a plurality of identical transverse perforated areas wherecross-section decreases progressively in a direction longitudinally ofsaid ribbon to a line of minimum cross-section and Where cross-sectionincreases progressively in a direction longitudinally of said ribbonwith increasing distance from said line of minimum crosssection; and anoverlay of a metal having a relatively low fusing point on said ribbon,said overlay extending from a region remote from said line of minimumcross-section substantially to said line of minimum cross-section butnot ybeyond said line of minimum cross-section whereby both relativelysmall and relatively large excess currents are caused to have adestructive eiect on said metal of which said ribbon is made `startingat said line of minimum Cross-Section- 7. A fusible protectivedevice'comprising a tubular casing of insulating material; a pair ofterminal elements closing the ends of said casing; a pair of bladecontacts extending in a direction longitudinally of said casing eachprojecting through one of said pair of terminal elements from theoutside of said casing into the inside thereof; a ribbon of copperhaving a Width approximately equal to the width of said pair of bladecontacts conductively interconnecting the axially inner ends thereof; aplurality of identical spaced parallel lines formed of circularperforations each extending transversely across said ribbon and eachdefining a transverse line of minimal crosssectional area; and atransverse overlay on said ribbon of va metal having a fusing pointlower than the fusing point of copper, said overlay -being substantiallyequal in width to the Width of said ribbon, v and said overlay having aplurality of projections extending substantially in a directionlongitudinally of said ribbon jutting into the spaces formed between theconstituent perforations of one of said lines to the points of minimalcross-section formed by said spaces but not significantly beyond saidpoints of minimal cross-section.

References Cited in the tile of this patent UNITED STATES PATENTS2,474,988 Sargrove July 5, 1949 2,493,434 Yonkers Ian. 3, 1950 2,832,868Kozacka Apr. 29, 1958 2,866,040 Skeats Dec. 23, 1958 v2,876,312Frederick Mar. 3, 1959

